Essential Water (VCE SSCE Chemistry): Revision Notes

Essential Water

Introduction

Water possesses unique chemical characteristics that make it essential for life on Earth. It regulates our weather patterns, shapes landscapes, and supports all living organisms. Water is the most abundant liquid on our planet, covering over 70% of Earth's surface. The total water supply on Earth exceeds 1.3 billion cubic kilometres and continuously cycles between land, oceans, rivers, and the atmosphere.

The availability of drinking water

In Australia, we have reliable access to clean drinking water through taps in our homes, schools, and workplaces. However, approximately 780 million people worldwide lack access to basic drinking water. Although Earth contains vast quantities of water, the ability of water to dissolve many substances means that many natural water sources do not provide fresh, drinkable water.

Water sources on Earth

Water exists naturally on Earth in three states of matter: solid (ice), liquid (water), and gas (water vapour).

The distribution of water across different locations on Earth is shown below:

Location of water	State of matter	Volume (km³)	Percentage of total water (%)
Oceans	Liquid	1,300,000,000	96.54
Ice caps and glaciers	Solid	24,000,000	1.74
Groundwater	Liquid	23,000,000	1.69
Ground ice and permafrost	Solid	300,000	0.022
Lakes	Liquid	180,000	0.013
Soil moisture	Liquid	17,000	0.001
Atmosphere as water vapour	Gas	13,000	0.001
Rivers	Liquid	2,100	0.0002

Despite these large quantities of water, there is a limited supply of fresh water on our planet. Only 2.5% of Earth's water is fresh and therefore potentially drinkable. Most of this fresh water is not accessible because it is locked up in ice caps, glaciers, or soil.

Key point: Of the 2.5% fresh water on Earth, only 0.5% is accessible. Within this accessible water, freshwater lakes contain 54%, groundwater contains 38%, and the atmosphere contains 8%.

Obtaining clean drinking water in Australia

Potable water is clean drinking water that has been treated to make it safe for human consumption. Obtaining potable water requires significant infrastructure to supply towns and cities with high-quality drinking water from naturally occurring water sources.

Major sources of drinking water

Drinking water in Australia comes from various sources:

• Reservoirs filled by run-off from rivers and streams
• Water obtained directly from rivers and lakes
• Groundwater (often called bore water in Australia)
• Recycled water
• Desalinated seawater

In most Australian major cities, water comes from reservoirs built on rivers in protected areas. These reservoirs are surrounded by protected land, often located in national parks and forests with limited access to ensure water quality remains very high. Water from such sources remains clean and requires only minor treatment before consumption.

In some inland parts of Australia and other Asia-Pacific countries, water comes from sources other than protected catchments. These may include rivers and lakes subject to contamination from agricultural and urban run-off, or groundwater sources that may require more complex purification processes.

Drinking water in Victoria

Most Victorians obtain drinking water from a mains water supply - water piped from a reservoir and controlled by a local water authority. Where piped water supply is unavailable, drinking water may come from:

• Rainwater tanks
• Bores
• Dams
• Rivers and creeks

The Great Artesian Basin

The Great Artesian Basin is the world's largest underground water supply (artesian basin), lying beneath one-fifth of the Australian landmass. It provides a reliable water source for irrigation, livestock, and domestic use across a large part of inland Australia.

Key features:

• Covers over 1,700,000 km², underlying nearly one-fifth of Australia
• In some places, the basin extends up to 3,000 m deep
• Water temperatures range from 30°C to 100°C
• Traditionally accessed through bore holes, now managed by government initiatives to maintain water stores

Water contamination risks

Different water sources present different levels of contamination risk. The hierarchy from lowest to highest risk is:

Special properties of water

Water possesses several special properties that allow it to support life. Space scientists search for water throughout the universe as an indicator of potential life beyond Earth. These special properties can be explained by understanding the structure of water molecules and the hydrogen bonding between them.

Structure and bonding of water

Water has the chemical formula $\text{H}_2\text{O}$, meaning each water molecule contains one oxygen atom covalently bonded to two hydrogen atoms.

Hydrogen bonding

The main type of intermolecular force between water molecules is the hydrogen bond. Hydrogen bonds form through electrostatic attraction between the partial positive charge on a hydrogen atom of one water molecule and a non-bonding pair (lone pair) of electrons on the oxygen atom of a neighbouring water molecule.

The large electronegativity difference between oxygen and hydrogen atoms creates relatively large partial charges on the atoms in a water molecule. This makes the electrostatic attraction between opposite partial charges strong, resulting in relatively strong hydrogen bonds between water molecules.

Relatively high melting and boiling points

Compared to other molecules of similar size, water has exceptionally high boiling and melting points. This is most clearly demonstrated by comparing water with other group 16 hydrides.

Group 16 elements include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Each can bond with hydrogen to form a hydride compound. Water can be classified as a group 16 hydride (it could also be called hydrogen oxide).

Group 16 element	Name of hydride	Formula of hydride
O	Water	$\text{H}_2\text{O}$
S	Hydrogen sulfide	$\text{H}_2\text{S}$
Se	Hydrogen selenide	$\text{H}_2\text{Se}$
Te	Hydrogen telluride	$\text{H}_2\text{Te}$
Po	Hydrogen polonide	$\text{H}_2\text{Po}$

The group 16 hydrides are all molecular compounds. Their melting and boiling points reflect the strength of intermolecular forces between molecules. Higher melting and boiling points indicate stronger intermolecular forces, as more energy is required to overcome these forces and allow molecules to move apart during melting or evaporation.

Physical properties of group 16 hydrides:

Hydride	Molar mass (g mol⁻¹)	Melting point (°C)	Boiling point (°C)
$\text{H}_2\text{O}$	18.0	0	100
$\text{H}_2\text{S}$	34.1	-82	-60.7
$\text{H}_2\text{Se}$	81.0	-66	-41.5
$\text{H}_2\text{Te}$	129.6	-49	-2.2
$\text{H}_2\text{Po}$	212.0	-35	36.1

Apart from water, both melting and boiling points increase down the group of elements. The intermolecular forces strengthen down the group as the atom bonding with hydrogen increases in mass. For example, $\text{H}_2\text{Po}$ has greater mass than $\text{H}_2\text{S}$, $\text{H}_2\text{Se}$, and $\text{H}_2\text{Te}$, giving it stronger dispersion forces and higher melting and boiling points. However, $\text{H}_2\text{O}$, which has the smallest mass, has the highest melting and boiling points.

Water also has significantly higher melting and boiling points than other molecular substances of similar size, including neon (Ne), hydrogen fluoride (HF), ammonia ($\text{NH}_3$), and methane ($\text{CH}_4$).

Density of liquid water and solid ice

Another unusual property of water is that solid ice floats in liquid water because solid ice has lower density. For most other substances, the solid state is denser than the liquid, causing the solid to sink in the liquid.

Density is a measurement of the mass of a unit volume of a substance, calculated using:

$\text{density (g cm}^{-3}\text{)} = \frac{\text{mass (g)}}{\text{volume (cm}^3\text{)}}$

In solid ice, there is less mass present in the same volume compared to liquid water. Water molecules are arranged in ice in a more spread-out pattern than in the liquid state. Solid ice has a density of 0.917 g mL⁻³, whereas liquid water has a density of 0.997 g mL⁻³ at 25°C.

Expansion of water on freezing

When water freezes, it expands. A sample of liquid water occupying 100 mL in a measuring cylinder expands to approximately 110 mL when frozen. This expansion can even crack containers if they are not designed to accommodate it.

Molecular explanation: As liquid water cools, water molecules move more slowly. When approaching the freezing temperature, molecules arrange so that each water molecule forms four hydrogen bonds with four neighbouring water molecules. This creates a very open arrangement of molecules, meaning water molecules are more widely spaced in ice than in liquid water. Therefore, ice is less dense than liquid water and floats. When ice melts, water molecules move more freely and become closer together.

The crystal lattice structure of ice shows how each water molecule connects to four others through hydrogen bonds, creating a rigid framework with significant empty space between molecules. This open structure explains why ice is less dense than liquid water.

The importance of floating ice

The fact that ice floats has critical ecological importance. In countries with cold winters (such as parts of North America, the United Kingdom, and Europe), lakes and rivers completely freeze over. While this can be inconvenient for navigation, requiring icebreaker ships to break through surface ice, the low density of ice is essential for the survival of aquatic organisms.

A layer of ice on the surface of water bodies forms an insulating barrier that separates warmer water below from cold air temperatures above. If ice were denser than water and sank to the bottom, new surface water would become exposed to cold air, also freeze, and sink. Eventually, the entire lake or river could freeze solid from bottom to top, and living organisms would not survive. Water below ice layers maintains a temperature of approximately 4°C, which is cold but above freezing, allowing aquatic life to survive.

The density of liquid water varies slightly with temperature:

Key observation: Liquid water reaches its maximum density at approximately 4°C. At temperatures both above and below 4°C, water is slightly less dense. This property is crucial for aquatic ecosystems, as the densest water sinks to the bottom while ice forms at the surface.